Electrical measuring instrument



May 9, 1939. FV. E. wENGER 2,157,973

ELECTRICAL MEASURNG INSTRUMENT Filed Oct. l2, 1957 an/nvaw/v /sm/wfUNA/Ecrin HERE BATTERY D.C.50URCE v 'ON l l Patented May 9, 1939 PATENTOFFICE ELECTRICAL MEASURING INSTRUMENT Floyd E. Wenger, Bluffton, Ohio,assignor to Ray L. Trlplett, Bluffton, Ohio Application October 12,1937, Serial'No. 168,643

4' claims.

l5 ing the zero position or the meter, in order to compensate forvariations in the operating current without changing the zero adjustmentoi the meter when the range thereof is changed.

In the drawing:

' Figure 1 is a wiring diagram of a multiple-range ohmmeter circuit witha single zero adjuster, the operating current for the circuit beingobtained by rectifying alternating current.

Figure 2 is a circuit similar to Figure 1, employing a batterylor otherdirect current source for the operating current.

In general, the electrical measuring instrument of this inventionconsists oi a source of operating current, either direct current orrectied alternating current, connected to a voltage divider having avariable resistor associated therewith, and a voltmeter calibrated inohms connected to taps taken oir of the winding of the voltage divider.The adjustment of the single variable resistor serves to compensate forvariations in the operating current, and enables thezero of thevoltmeter to be reset without disturbing the adjustment for otherranges. t

Hitherto, in multi-range ohmmeters a zero 4o adjusting device has beenprovided for each range or else the adjusting means has requiredreadjustment when changing ranges. The present invention eliminatesthese multiple zero adjustments and also eliminates the disadvantage oichanging the zero settings for each range by providing a singleadjustment. which is eiiective for all ranges. v

Referring to the drawing in detail, Figure 1 shows'a multi-rangeohmmeter circuit including a. positive current input line III connectedto'a voltage divider II, having taps I2, I3, I4 and Iltaken oir atvarious points thereon, these taps including series resistors I8. I1, I8and I8. The number of taps may be any'convenient number other than v thefour taps shown in the wiring diagram. The

range tap switch arm 20 selectively engages'the taps I2 to I5,inclusive, and is connected through the line 2| to the positive terminal22 oi the voltmeter 23, the negative terminal thereof being connectedthrough the line 25 to the measuring terf8. minal 26. The oppositemeasuring terminal 21 is grounded through the line 28, as for example,to the instrument case at 28. The switch 38 serves to short-circuit themeasuring terminals 28 and 21 at the time when zero 'adjustments of theinl'l0 strument are made. During measurements, however, the switch 30 isopened and the unknown resistance is connected across the measuringterminals 26 and 21. Connected to the opposite end of the voltagedivider II from the positive current l5 input line I0 is the line 3lleading to the variable resistor 32, having the slider 33 grounded, as

at 34, such as to the instrument case.

In Figure 1 the positive current input line Il and the negative currentinput line 28 receive cur- 20 rent from an alternating currentrectifylng circuit. This circuit consists of the alternating currentpower lines 35 and 36, having a power switch 31, these lines beingconnected to the primary winding 38 of the transformer 39. 'I'hesecondary 25 winding 4I) of the latter is connected by the lines 4I and42 to the filament 43 of the full-wave rectifying tube 44, the plates 45and 46 thereof being connected by the lines 41 and 48 to the secondarywinding 49 of the transformer 39. The second- $0 ary winding 49 isgrounded by the line 50 running from an intermediate point on thewinding to the ground at 5I; such as on the instr'ument case. From thelament line 4I the line 52 runs to the iilter choke 53, the opposite endof which is con- 35 nected to the positive current input line I0. Theopposite ends of the lter chokev 53 are connected to the ltercondensers54 and 55, the opposite sides of which are grounded as at 58 and 51.

In the arrangement shown in Figure 2 the posi- 40 tive current inputline I0 is connected to the positive pole 58 of a battery or otherdirect current source 59, the negative pole 60 oi' which is connected bythe line 6I to the ground or instrument case, as at 62. '45

In the operation of the circuit of Figure 1 the power switch 31 isclosed, thereby connecting the primary winding 38 of the transformer k38to the alterantlng current mains. This causes a voltage to be appliedacross the plates 45 and 48 5 and lament 43 of the rectier tube 4.4.This voltage is rectied in the rectifier tube 44 and ilo'ws from theiilament 43, through the line 82, to the filter choke 53, where it issmoothed out by the mier choke s: and the nite;- condensem' l input lineIII, the voltage divider Il and 55.' The current, which is of a'positive character, continues through the positive current II, the line3|, the variable resistor 32, the slider 33 and the ground connection 3lto the ground connection 29, which is also connected to the center tapground connection 5I of the transformer secondary winding I9, therebycompleting the current path.

In the circuit of Figure 2 the course of the current is similar,exceptthat it proceeds directly from the positive pole 58 of thebattery, or other direct current source 59, through the positive currentinput line, along the route previously followed, f the circuit beingcompleted at rthe ground connection 92 and line 6I leading to the'negative pole CII of the battery, or other current source 59.

Each of the taps I2 to I5, inclusive, oi' the voltage divider II iscalibrated to give a definite voltage drop so that there appears acrosseach tap the required voltage for the operation of the meter 23 as anohmmeter for the diilerent ranges To this end the taps of the voltagedivider II are adjusted very carefully in order to -give the correctvoltage drop. The function of the resistors I8 to I9, inclusive,connected in series with the taps I2 to I5, inclusive, of the'voltagedivider II, is to limit the current or drop in voltage which is used toactuate the meter 23 to such a value as will permit the meter 23 to readwith a full scale deiiectionior each range. i

For the first range the voltage available for operating the voltmeterappears between the tap I2 and the ground connection 29, the currentpassing from the tap I2,'through .the range tap switch 20, the line 2I,the meter 23, the line 25, through the switch during the preliminary'adjustments of the meter before the actual measurement of the unknownresistance is attempted; The closing of the switch 3l short-circuits theterminals 26 and 21 for all zero adjustments of the ohmmeter, andprovides for a continuity of circuit. Under these circumstances, thevalue of the current flowing through the meter 23 is determined by thevoltage and the current limiting resistor I6.

The value of the resistor I6 can be calculated from Ohms law. Forexample, when the voltmeter 23 consists of a 0-1 milliammeter of 100ohms, and the voltage drop from tap I2 on the voltage divider to tap I5is 120 volts, the value of the resistor I6 would be 119,900 ohms.according to Ohms law. If the setting of the variable resistor 32 isaltered by moving the slider 33, this changes the current which flowsthrough the voltage divider II, and inturn, changesthe current acrossthe taps I2 and I5. Under these circumstances, the meter 23 wouldread'some value other than the full scale. Ohm scalesV are usually designedwith ranges in multiples of ten. The next voltage step of the meterwould be I2, this appearing between the tap I3 of the voltage divider IIand the negative ground connection 29. This, for example, requires an11,900-ohms resistor I1 in series with the tap I3 engaged by the rangeas before.

In a `similar manner, using a'10 1 ratio, the voltage divider IIwould'bev adjusted sothat 1.2 volts will be developed across the tap I4of the voltage divider II and the ground connection, whereupon the themeter 23.

vupon-the current value will ydirectly in ohms.

30 and the line 2l to the f ground connectionk 29. The switch 30 isclosed Acorrect current minals, switching tap switch arm 20, and theaction of the meter 23 would be the same the tap I5 of the voltagedivider and the ground connection 29, with a proper adjustment oi' thetaps of the voltage divider II a voltage of 1.2 volts would be developedbetween the tap Il and the ground connection 29. whereupon the seriesresistor I9 would have a value of 20 ohms and the action would be thesame as described above.

If, now, the voltage between the line 52 and the ground connection 2,9changes, the current through the voltage divider II- would also change,thereby giving incorrect voltage drops through In order to compensatefor these current changes, the variable resistor 32 is advjusted bymoving the slider 33 so that the total voltage between the line 52 andthe ground connection 29 will be the same as that voltage forwhich theinstrument was calibrated, wherebe the same and the voltage drop acrossthe taps I2, I3, I4 and Il ofthe voltage divider II will be the sameduring calibration.

'When the zero o1y the lmeter 23 is properly adjusted by moving theslider 33 of the variable resistor 32, the switch 30 is opened and theunknown resistance is connected across the terminals 26 and 21. Thevalue of this unknown resistance in ohms is then read on the scale oi'the meter 23. As the unknown resistance reduces the current, the metercan be calibrated to read When the meter 23 has been adjusted to itszero position, the setting does not have to be changed until the voltageof the operatingj current changes. 'I'he zero setting will thus remaincorrect for the different ranges of the instrument. The instrument o1'this invention, therei'ore, enables the setting of amultiplerangeohmmeter to zero for all the ranges'oi' the instrument, provided thatthe taps of the voltage divider II have been very carefully Aadjustedinorder to obtain the correct voltage drop. The

iiowing through the voltage dividerII thus gives the correctvoltage'drop used to operate the meter 23.

magnitude oi' a resistance, a voltmeter, a range switch potentiometerresistor, and an adjustable resistor all connected in series with theresistance of unknown value, and means for applying a measuring voltageacross the range switch potentiometer resistor and the'adiustableresistor. measuring the unknown v 2. In a system for magnitude of aresistance, the combination of a source oi potential, an electricalmeasuring device, a iixed resistor provided with taps, a variableresistor., said ilx'ed and variable resistors being connected in serieswith said source, a plurality of resistances connected respectivelybetween said taps and-a corresponding plurality of termeans forselectively contacting said terminals, the electricalmeasuring device,the resistance ofunknown value and said variable resistor beingconnected in series with said switching means, said vvariable resistorbeing adapted to compensate for the fluctuations in .thenvoltage of saidsource.; series resistor I3 would have -a value oi 1100 ohms. yIn a likemanner, between I n a. system. for f measuring the unknown ,tude of aresistance, the combination of a source of potential, a fixed resistorprovided with taps, a variable resistor, and a voltmeter, said xed andvariable resistors being connected in series with said source, aplurality of range resistances connected respectively between said tapsand a corresponding plurality of terminals, switching means forselectively contacting said terminals, the voltmeter, the resistance ofunknown value and said variable resistor being connected in series withsaid switching means, said variable resistor being adapted to compensatefor uctuations in the voltage of said source, and said range resistanceshaving magnitudes such as to provide full scale deection on saidvoltmeter for encountered magnitudes of unknown resistance.

4. In a system for measuring the unknown magnitude of a resistance, avoltm'eter, means for selectively changing the measuring range of v saidvoltmeter, said means comprising a.plural ity of switch terminals and acorresponding plurality of resistors connected respectively to saidterminals, said resistors terminating in a common potentiometerresistor, an adjustable resistor connected to said common potentiometerresistor, Y

said volt'rneter, switch terminals, resistors, and adjustable resistorall being connected in sexies with the resistance of unknown ,magnitudemeans for applying a measuring voltage to said common potentiometerresistor, whereby said adjustable resistor serves to compensate foructuations in said voltage, and means for applying a known voltage tothe resistance of .unknown magnitude, said voltmeter serving totranslate the current flowing through the resistance of unknownmagnitude into ohmio measurement.

FLOYD E. WENGER.

